Seismic prospecting in aquatic mediums is conventionally performed by placing a set of parallel submerged seismic cables carrying a plurality of sensors (hydrophones or hydrophone/geophone combinations) spaced apart along the cable, the set of cables being towed by a seismic vessel. A source capable of creating an acoustic wave in the medium, generally in the form of an array of air guns, is towed by the seismic vessel at a distance from the seismic cables. The wave thus formed propagates to the water bottom, then further to the underlying subsoil, which gives rise to reflections at the interfaces between the geological layers forming the subsoil, which are collected by said submerged sensors. All of the information is then processed to produce a three-dimensional (3D) image of the different geological layers of the subsoil, generally used to determine the presence of hydrocarbon reservoirs.
Document FR2940838A1 discloses a seismic prospecting technique in which a set of parallel submerged seismic cables of neutral buoyancy are positioned midwater, and each cable is connected to respective surface autonomous recording vessels (hereinafter “RAVs”), sometimes called “RAVs”, at each of its two ends and subject to a tension exerted at its ends by the RAVs in opposite directions, so that the cables can be kept stationary or quasi-stationary during the acquisition of seismic signals. Various aspects related to this technique have been disclosed in FR2945356A1, FR2961317A1 and FR2990028A1.
The technique disclosed in document FR2940838A1 provides, in addition to other advantages presented in this document, great flexibility in the positioning of the seismic cables as compared with the conventional method in which the entire set of cables is towed by a seismic vessel; it enables in particular the cables to be positioned at a water depth much greater than the depth in the range of 5 to 15 meters which is a constraint of the aforementioned conventional method.
The present invention relates to a method that is easy to implement and improves the precision of the control of the position in a horizontal plane, i.e., a plane perpendicular to the vertical in the terrestrian reference base (hereinafter “horizontal position”) of a seismic cable adapted for acquiring seismic data midwater and kept stationary or quasi-stationary during the data acquisition.